A. Field of the Invention
This invention relates generally to the field of cameras used for aerial reconnaissance, aerial surveying, mapping and other applications in which the camera is placed onboard a moving vehicle. The invention also relates to a method of operating such a camera so as to obtain high aspect stereo imagery of the earth in a manner such that an increased amount of three-dimensional information can be obtained of the terrain of interest as compared to prior methods. The invention is particularly well suited to both framing camera and line scanning cameras, and cameras based on either film or electro-optical image recording media.
B. Description of Related Art
The traditional method for collection of stereo imagery using a reconnaissance camera provides for the capture of images in successive scan cycles with at least 50% forward overlap (56% is a typical value). Two pictures are taken of each point on the ground from two successive camera positions along the flight path. The imaging geometry is usually side oblique or vertical (camera pointed at nadir). In side oblique the azimuth pointing angle is at 90 degrees relative to the flight direction. Coverage across the line of flight (XLOF) is dependent on the size of the foot print of the field of view of the camera and the aircraft speed which also relates to the ratio of velocity to slant range at the near edge of the footprint. The aircraft position advances only one-half of the near edge length between successive frames, and the maximum stereo angle is one-half of the sensor field of view. This situation is shown in FIG. 1, where xcfx86s indicates the stereo angle. Pairs of images 1 and 2 can be recombined by means of a stereo viewing system to achieve depth perception. Such stereo images are often used by a photointerpreter to gather additional information about the scene.
The operation of a film-type framing camera in a stepping mode is known in the art. For example, the article entitled xe2x80x9cThe KS-146A LOROP Camera Systemxe2x80x9d, Thomas C. Augustyn, SPIE Proceedings Vol.9, Aug. 27-28 1981, paper 309-11 p.76, describes an automatic stepping mode in which the camera cycle rate is proportional to aircraft velocity, altitude and selected depression angle, to achieve 56% overlap for stereo viewing or 12% overlap for maximum flight line coverage. With the camera line of sight normal to the flight path, the scan head provides 1, 2, 4, or 6 lateral-step cycles. A similar stepping operation for a frame camera is described in the article entitled xe2x80x9cKS-127A Long Range Oblique Reconnaissance Camera for RF-4 Aircraftxe2x80x9d, Richard C. Ruck and Oliver J. Smith, SPIE Proceedings Vol. 242, Jul. 29-30, 1980 San Diego Paper 242-02, p.22. The contents of both articles are incorporated by reference herein.
An electro-optical step frame camera is described in U.S. Pat. No. 5,668,594 to Lareau et al., the content of which is incorporated by reference herein. In the Lareau et al. ""594 patent, a mirror is rotated in series of steps about an azimuthal axis to direct scene information in the cross-line of flight onto the image detector array. An image is obtained at each angular position. Forward motion compensation is performed on-chip. As the camera steps to a new angular position, the scene is read out. By repeating the stepping action in a manner in which an overlap exists between successive frames or between frames in a series of cycles, it is possible to obtain some stereo coverage.
Panoramic (pan) camera technology is another well-established means of imaging. In a panoramic scanning camera, the scene is exposed continuously by rotating a scanning mechanism (such as a double dove prism) so as to scan the image across the photosensitive medium. The photosensitive medium is moved in synchronism with the image. In the case of a film camera, this may be accomplished by moving the film at the same rate as the image rate past an exposure slit which is located on the lens optical axis. A scan prism located in front of the lens is rotated in synchronism with the film rate such that the image of the scene remains stationary on the film during the exposure period. The slit opening is adjusted to a predetermined width to control exposure time.
One major advantage of a pan camera is its ability to image a large area in the direction across the line of flight. Scan angles across the line of flight on the order of 120 to over 180 degrees are typical. The lens field of view in a pan camera is generally only required to be large enough to cover the width of the film. Overlapping of images and stereo imagery may also be obtained with pan cameras. Image overlap in a conventional fixed mounted pan camera is obtained as in the case of a framing camera, that is, in the common area between successive scans. The stereo convergence angle can be up to one half the field of view of the camera, depending on the forward overlap.
The stereoscopic imagery generated by the above methods suffers from a disadvantage, in that the baseline between adjacent stereo images and the associated stereo angle xcfx86s, are typically relatively small, resulting in xe2x80x9clow aspectxe2x80x9d stereo image pairs. Consequently, the imagery is only capable of generating minimal three-dimensional information. The art has lacked a method for obtaining stereo pairs that have a high baseline (xe2x80x9chigh aspectxe2x80x9d, as that term is used herein) and thus generate a high degree of three-dimensional information about the terrain of interest. The present invention meets that need. Another disadvantage of the prior art is that in order to capture a stereo image of all points in the scene, three images must be taken of that scene and combined when using the typical 50-56% overlap. See FIG. 1A.
Other patents of interest include Busenberg, U.S. Pat. No. 5,251,037; Hiramatsu, U.S. Pat. No. 5,138,444, and Lareau et al., U.S. Pat. No. 5,155,597. Hiramatsu provides a technique for obtaining stereo pairs of imagery of the earth but requires two camera units, one looking in the forward direction and one looking in the aft direction. Furthermore, the drawings and the complexity of the associated circuitry used by Hiramatsu suggest the use of line scan cameras. The complexity is created by the need for the location and registration of scanned lines of imagery. The preferred embodiment of the present invention removes that complexity by utilizing a framing camera. The Lareau patent describes how a electro-optical framing camera can take overlapping frames of imagery to obtain some stereo coverage (like the situation shown in FIG. 1), but the images are not at high aspect since the amount of overlap is limited by the field of view of the camera.
Advantages of the present invention over prior art methods can be summarized as follows:
1. The high aspect stereo coverage of the scene yields much more three dimensional information as to the terrain and objects in the scene as compared to prior art methods. Mensuration techniques can be applied with more accuracy.
2. This invention requires lower complexity due to the use of a single camera vs. the two cameras required by Hiramatsu. There is no requirement for the use of correlation methods to find the stereo complement of a given frame.
3. The stereo angle is variable and can be selected by the operator to accommodate operational constraints such as velocity to slant range ratio or field of regard limitations of the installation.
4. The approach yields wide-angle stereo coverage without a requirement to use a short focal length camera, thus preserving system scale (photographic scale).
5. The achievement of high aspect stereo with a single camera avoids the need to match the lens focal lengths of two or more cameras.
6. The ability to point the Line of Sight (LOS) for the second stereo picture allows for maximum frame overlap between stereo pairs, rather than requiring three frames to cover all points in stereo from a single picture as is required by traditional stereo photography.
7. Fore-aft movements can be done during the inter-frame period, so no additional time is required to image the terrain.
A new operating mode or method for cameras used in aerial photography is described which allows for generating high aspect stereo imagery of a terrain of interest with a single camera. This mode of operation is available in any camera with fore-aft pointing control. The capability is not dependent on the camera being an E-O camera, since the same effect can be achieved with a film camera.
The method provides for generating a first image with the camera angle rotated about an azimuthal axis some angular amount (xcex81) to a first, typically forward orientation, i.e., forward of a cross-line of flight direction. An image is obtained of the ground at this orientation. Then the camera is rotated about the azimuthal axis to new angular value (xcex82), which will typically be aft of the cross line of flight direction. An image is generated at this value. The camera is then rotated back to the value of xcex81 (or xcex81xc2x1some value xcex5), and a second image in the forward orientation is generated. The camera is then rotated again to the value of xcex82 (or xcex82xc2x1some value xcex5) and another image in the aft orientation is generated. This process of rotating the camera about the azimuthal axis and generating images in forward and aft orientations continues over and over. Eventually, as the aircraft flies past the terrain of interest, any given location in the terrain of interest will have been imaged from two different perspectivesxe2x80x94forward and aft. The motion of the aircraft during the interim in combination with the values of xcex81 and xcex82 provide the high baseline for the stereo image pairs. By selection of suitable values for the angular separation of xcex81 and xcex82 (such as xcex81=+10 degrees and xcex82=xe2x88x9210 degrees) (xcfx86s=20 degree) the result will be pairs of images of the terrain of interest having a large baseline, producing truly high aspect stereo images from a single camera.
As an alternative implementation, the camera can be oriented or pointed relative to the aircraft to generate images about nadir in the vertical forward oblique and vertical aft oblique and the camera pointing device is repeatedly rotated to two angular positions about the aircraft pitch axis. The camera obtains high aspect stereo pairs of images as the aircraft flies directly over the terrain of interest.
While the camera is operating in the side oblique mode (or vertical mode) at an any given azimuthal orientation, one alternative embodiment provides for stepping in the cross-line of flight direction by changing the camera depression angle in a series of incremental movements or positions (xe2x80x9cstepsxe2x80x9d) and generating a frame of imagery at each position. The xe2x80x9cstep-framexe2x80x9d operation of the camera, combined with the rotation of the camera about the azimuthal axis, generates both wide scene coverage in the cross line of flight direction and high aspect stereo images of the terrain.
In another preferred embodiment, the camera is operated in a mixed mode. In addition to operating in a given side oblique or vertical orientation and generating high aspect stereo by looking forward and aft, and in addition to combining this mode with step framing to capture a wide area coverage as described above, the camera may also combine the generation of high aspect stereo with non stereo single frame imagery as part of the reconnaissance mission. In other words, the camera can be rotated to intermediate values of xcex8 (such as xcex8=0, i.e., cross line of flight direction) and images can be taken at that orientation as well. Furthermore, at any given value of xcex8, the camera can scan across the line of flight, generate a single frame of image, generate a plurality of frames at different depression angles, or image the terrain in other modes, depending on the type of camera and the requirements of the reconnaissance mission.